WO02/01250 discloses the use of oxymetal salts in laser marking. AOM (ammonium octamolybdate) is an example of a material that can be marked directly with 10,600 nm laser radiation. However, the use of a NIR (near infra-red) laser would offer additional advantages.
WO05/068207 discloses the use of NIR laser radiation (i.e. at 800 to 2000 nm) to initiate a colour change reaction when a NIR-absorbing metal salt is used in combination with a substance that normally undergoes a colour change reaction at much longer wavelength (˜10,600 nm), e.g. AOM. A plethora of metal salts are described; particularly preferred salts are of copper, e.g. CHP (copper hydroxy phosphate).
WO05/095516 discloses that r-ITO (reduced indium tin oxide), a non-stoichiometric compound, can be incorporated into thermoplastics which are then heated, using non-laser NIR heating lamps, above their glass transition temperatures for stretch blow-moulding, e.g. in the production of PET beverage bottles from injection-moulded performs. r-ITO is particularly suitable for this purpose as it is mostly transparent in the visible region.
JP8127670 discloses the use of reduced titanium oxide compounds for incorporation into thermoplastics, for the laser marking of moulded products.
U.S. Pat. No. 5,578,120 describes the use of a laser beam-absorbing inorganic substance and a colorant. Examples of the colorants that can be used include those mentioned in JP-A-49-82340, e.g. zinc oxide semiconductors and titanium dioxide semiconductors.
U.S. Pat. No. 5,911,921 discloses the use of non-stoichiometric ytterbium phosphate to produce NIR absorbing inks, e.g. for printing stealth bar codes.
U.S. Pat. No. 6,602,595 discloses the use of non-stoichiometric nano-materials for use in inks. r-ITO is mentioned as an NIR absorber.